Prior solid state charged particle detectors, used in transuranic aerosol monitor designs, are provided with annular or open-flamed inlets through their detector housings which serve as entrance windows in order to permit charged particles to have access to the active surfaces of the detectors and to improve sample collection efficiency. However, charged particle detectors having such entrance windows are sometimes subject to unacceptable susceptibility to radio frequency interference and other electronic noise due to leakage of spurious energy into the detector housings through the entrance windows.
Metalized film barriers may be used to provide shielding over the detector entrance windows in order to provide enhanced protection against the electronic noise. However, charged particle detectors having this method of enhanced protection are of limited application because of the associated loss of resolution of detected charged particle energies resulting from the layer of metalized film shielding. Typically, the loss of resolution is sufficient to make the distinction between transuranic isotopes and the naturally occurring radon decay products difficult to determine at the lower particle concentrations of interest.
For example, prior art charged particle detectors are typically used to detect a range of alpha particle energies from 4.2 to 5.2 Mev resulting from the original decay energies of 5.2 to about 5.5 Mev from transuranics. If alpha particles must pass through a metalized film layer, in addition to an air gap, the range of alpha particle energies striking the active surface of the detector will be considerably broader. With increased degradation of alpha particle energies, many of those originating at typically 6 and 7.68 Mev from the naturally occurring decay products of Radon will be indistinguishable at the detector surface from those originating from transuranic elements.
Another problem with the prior art is the occurrence of ground loops in the detector housings. Such ground loops result from the high frequency skin effect currents through the detector housings caused by electromagnetic radiation, typically associated with communication signals and electrical equipment operation, and the resistance inherent in the detector housings. Current in ground loops created in this manner may cause spurious signals within the detector circuitry similar in amplitude to alpha particle interaction signals and cause inaccurate detection results.
The present invention includes a thin conductive coating of approximately one thousand angstroms over a silicon solid state detector which has a polymer insulating layer to provide resistance to abrasion and chemical corrosion. The conductive coating provides an isolated shielding layer with a minimum loss in detector charged particle energy resolution. The use of beryllium for the conductive shielding layer is preferred over other metal coatings such as aluminum or gold. A metallic detector support structure is electrically connected to the conductive coating in order to provide a continuous electronic shield. The inner surface of the detector support structure is lined with a radio frequency absorbing material such as ferrite in order to minimize conductive and reflective effects in the support structure in the vicinity of the active surface of the detector. The internal frequency absorbing lining allows the exterior of the assembly to be a smooth, cleanable surface suitable for aerosol monitoring or related applications.